A fuel cell converts chemical energy directly into electrical energy. Most fuel cells comprise a cathode or air electrode 1 and an anode or fuel electrode 3, separated by an electrolyte 5 (FIG. 1). At the air electrode, oxygen is ionized and the oxide ions migrate through the electrolyte to the fuel electrode 3. At the fuel electrode 3, hydrogen is ionized and the hydrogen ions react with the oxide ions to form water and release electrons. The released electrons then travel from the fuel electrode 3 to the air electrode 1 through a load-containing connection, thereby completing the circuit and providing a small amount of direct electrical current. It is well known in the art that ion quantities can vary, additional or other ion constituents can be used, and ion and electron directions can be reversed.
A fuel cell based power generation system typically comprises a plurality of electrically interconnected fuel cells. The system usually uses a hydrogen-bearing and/or carbon-bearing fuel (e.g. natural gas, methane, carbon monoxide, hydrogen) at the anode, and an oxidant (e.g. air, oxygen) at the cathode. A schematic arrangement of one such system, which uses solid oxide electrolyte fuel cells (SOFC), is described in U.S. Pat. No. 4,395,468.
Because fuel cells are efficient, use plentiful and renewable fuels, do not require direct combustion and produce low emissions, they are a very attractive energy source. However, although the basic electrochemical processes and schematic arrangement of fuel cell based power generation systems are well understood, engineering solutions necessary to lower fabrication costs and make such systems an economical alternative to fossil fuel and other power generation systems remain elusive.
One technical problem with conventional fuel cells involves the application of the fuel electrode to the electrolyte. The applied fuel electrode should advantageously possess and maintain certain properties during a lifetime of operation under fuel cell operating conditions with various fuels, including varying temperatures (e.g. about 25-1200° C., preferably about 700-1000° C.) and pressures (e.g. about 0.5-5 atm, preferably about 1-5 atm). These properties include: high electrical conductivity, large electrochemically active interface area, high porosity, strong adherence to the electrolyte and interconnect, good chemical and physical stability, thermal cyclability, low fabrication costs, and long useful life.
One popular type of fuel electrode composition is a nickel-zirconia cermet (ceramic-metal mixture) such as those described in U.S. Pat. Nos. 4,597,170, 4,609,562, and 4,847,172. A successful process used to apply a nickel-zirconia fuel electrode onto an underlying electrolyte substrate involves the electrochemical vapor deposition (EVD) of yttria-stabilized zirconia within and surrounding nickel particles, thereby forming a yttria-stabilized zirconia “skeleton” within and around a matrix of nickel particles, such as described in U.S. Pat. No. 4,582,766. This process produces a fuel electrode that can generally meet the above-described technical properties, but which is quite expensive and time-consuming to manufacture. For example, such a process requires application of a room temperature nickel slurry followed by a costly high temperature EVD process.
In an effort to reduce fuel electrode manufacturing costs, sintering processes have been attempted, such as those described in U.S. Pat. Nos. 4,971,830, 5,035,962, 5,908,713 and 6,248,468. However, fuel electrodes applied by a sintering process are relatively time consuming in that it still requires at least two processing steps, an initial application followed by high temperature sintering. Moreover, sintered fuel electrodes may experience marginal physical stability over time.
Other attempts to reduce fuel electrode fabrication costs include plasma spraying (e.g. atmospheric plasma spraying “APS”, vacuum plasma spraying “VPS”, plasma arc spraying, flame spraying) which generally involves spraying a molten powdered metal or metal oxide onto an underlying substrate surface using a plasma thermal spray gun to form a deposited layer having a microstructure generally characterized by accumulated molten particle splats. Plasma spraying techniques are described in U.S. Pat. Nos. 3,220,068, 3,839,618, 4,049,841, and U.S. Pat. Nos. 3,823,302 and 4,609,562 generally teach plasma spray guns and use thereof, each of which are herein incorporated by reference in their entirety. Although plasma spraying has been used for fabrication of certain fuel cell layers, such as those described in U.S. Pat. Nos. 5,085,742, 5,085,742, 5,234,722 5,527,633 (plasma sprayed electrolyte) U.S. Pat. No. 5,426,003 (plasma sprayed interconnect), U.S. Pat. No. 5,516,597 (plasma sprayed interlayer) U.S. Pat. No. 5,716,422 (plasma sprayed air electrode) and Invention Registration No. H1260 (plasma sprayed air electrode, electrolyte and fuel electrode), use of such plasma spraying techniques have been of limited value when used to apply a fuel electrode onto an electrolyte because they tend to result in a fuel electrode that poorly adheres to the electrolyte and exhibits poor thermal cyclability due to the mismatch of thermal coefficients of expansion between the metal portion of the fuel electrode and the ceramic electrolyte. Moreover, these conventional plasma spraying techniques tends to result in a fuel electrode that has a low porosity after continued use, thereby causing voltage loss when current flows as a result of polarization due to a low rate of diffusion of fuel gases into and reaction product out from the interface between the fuel electrode and electrolyte.
There is thus a need for a fuel electrode and a method for making the fuel electrode that can generally achieve above-described favorable technical properties and can be applied onto an underlying electrolyte at a low cost.